Laser patterned C-V dot

ABSTRACT

A semiconductor capacitor used to test for contaminants in a fabrication line is created by: forming a layer of insulating material on a semiconductor substrate, forming a layer of conductive thin film on the layer of insulating material, and laser patterning an area of the conductive thin film. Laser patterning is performed by applying the laser along the outer boundary of the area to be patterned to energetically remove the conductive thin film along this boundary.

FIELD OF THE INVENTION

This invention relates to the laser patterning of a conductive thin filmin a semiconductor device. Such a conductive thin film can includemetals, conductively doped polysilicon and metal silicides.

DESCRIPTION OF THE PRIOR ART

Patterned areas of conductive thin film have previously been used tocreate semiconductor devices such as capacitors. A semiconductorcapacitor can be fabricated by forming an insulating layer on a siliconsubstrate and then forming an area of thin conductive material on top ofthe insulating layer. This area of thin conductive material is known asa capacitance-voltage (C-V) dot. Silicon dioxide is a commonly usedinsulating material, while typical thin conductive materials includeconductively doped polysilicon, metals and metal silicides.

When a voltage is applied to the C-V dot, the device acts as acapacitor. That is, charge carrying particles are forced across theinsulating layer such that the substrate contains a majority of chargeshaving one polarity and the C-V dot contains a majority of chargeshaving the opposite polarity. For this to occur, the C-V dot must beseparated from the substrate by the insulating layer.

One application of a C-V dot is the detection of contaminants, such assodium, in a wafer fabrication line. As a method of qualifying adiffusion furnace or other production equipment for volume productionusage, a layer of uncontaminated oxide is formed on a wafer. The waferis then run through the production equipment to be qualified so that thewafer is exposed to any contaminants present. Any contaminants introducecharge-carrying mobile ions into the wafer. A C-V dot is formed on theoxide by depositing metal through a mask or by using a photolithographicmethod to etch an area of deposited metal. A voltage supply is thenconnected to the C-V dot, and the applied voltage is cycled betweenpreselected maximum and minimum voltages. The capacitance is measuredfor the varying voltages. These measurements are used to produce acapacitance-voltage (C-V) plot, such as the one shown in FIG. 1. Curve 1in FIG. 1 shows the response of an ideal uncontaminated capacitor as theapplied voltage is cycled between -5 volts and 5 volts.

The charge-carrying mobile ions present in a contaminated capacitor willcause the capacitance to change. This is because the extra chargecarriers present in the wafer allow charge to flow more freely when theapplied voltage is one polarity and less freely when the applied voltageis the opposite polarity. Such a change in capacitance is illustrated bythe offset 3 in curve 2 of FIG. 2. The presence of offset 3 thereforeindicates the presence of contaminants in the production equipment.

Conductive thin films in semiconductor devices have previously beenpatterned by using one of two methods. In one method, an area ofconductive thin film is patterned by depositing the thin film materialthrough an opening in a metal mask physically attached to the wafersurface. However, this method has several undesirable characteristics.First, the metal mask may be very difficult to attach to the wafer orload into the metal deposition system. Additionally, scratches, particlecontamination, and chemical contamination may be introduced whenattaching the metal mask to the wafer. Another drawback is that themetal mask may only be used several times before it must be thrown away.Also, if contamination of the metal mask occurs, subsequent wafers canbe contaminated. Finally, the metal mask may fall off the wafer duringmetal deposition causing wafer breakage and/or downtime of the metaldeposition system.

In the other method, an area of conductive thin film is patterned usingphotolithography. First, an area of conductive thin film larger than thedesired patterned area is deposited on an upper surface of asemiconductor device. A layer of photoresist is deposited on the uppersurface of the conductive thin film layer. The photoresist layer is thenselectively exposed to ultraviolet light through a metal photomaskhaving opaque and transparent areas in the shape of the desired pattern.Depending on whether the photoresist is positive or negative type,either the exposed photoresist portions or the unexposed portions areremoved with a developer solution, leaving a mask which covers thedesired patterned area of conductive thin film. An etchant, whichremoves the conductive thin film around the desired patterned area, isthen applied to the upper surface of the photoresist layer. Thephotoresist layer is then removed, leaving the desired patterned area ofconductive thin film.

This method is undesirable since it requires many processing steps andintroduces risk of contamination during each of these steps. For thesereasons, photolithographic methods for creating patterned areas ofconductive thin film are not commonly used.

It would therefore be highly desirable to have a simple method forcreating patterned areas of conductive thin film which reduces the riskof contamination and eliminates the need for a metal mask orphotolithography.

SUMMARY OF THE INVENTION

The present invention provides such a method for patterning a conductivethin film.

It is one object of the present invention to reduce the risk ofcontamination when creating a patterned conductive thin film.

It is another object of the present invention to provide improvedaccuracy in creating a patterned conductive thin film.

It is a further object of the present invention to eliminate the needfor consumable metal masks in creating a patterned conductive thin film.

It is still another object of the present invention to eliminate thepossibility of downtime of a metal deposition system in creating apatterned conductive thin film.

In accordance with the present invention, a layer of insulating materialis formed on a semiconductor substrate. A layer of conductive thin filmis formed on top of this insulating material. A laser is then used topattern a closed area of the conductive thin film by energeticallyremoving the conductive thin film along the boundary of the desiredarea, thereby creating a semiconductor capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a Capacitance-Voltage plot for an ideal uncontaminatedsemiconductor capacitor;

FIG. 2 is a Capacitance-Voltage plot for a contaminated semiconductorcapacitor;

FIG. 3 is a cross section of a semiconductor capacitor before laserpatterning;

FIG. 4 is a perspective view of a semiconductor capacitor after laserpatterning has been performed to create a Capacitance-Voltage dot;

FIG. 5 is a cross section of the layers shown in FIG. 4 along sectionalline A--A;

FIG. 6 is a Capacitance-Voltage plot for a laser patterned C-V dot;

FIG. 7 is a summary of the measurements shown in the Capacitance-Voltageplot of FIG. 6;

FIGS. 8 and 9 are perspective views showing processing steps of analternate embodiment of the present invention; and

FIG. 10 is a cross section of the layers shown in FIG. 9 along sectionalline B--B.

DETAILED DESCRIPTION OF THE INVENTION

The present invention involves the patterning of thin layers ofconductive material in a semiconductor device using a laser.

In one embodiment of the present invention, a laser is used to pattern athin layer of conductive material to create a C-V dot. FIG. 3 shows across section of a monocrystalline semiconductor substrate 10, aninsulating layer 12, and a conductive thin film 14, prior to patterning.The semiconductor substrate 10 and insulating layer 12 are typicallysilicon and silicon dioxide, respectively. The conductive thin film 14is typically metal, conductively doped polysilicon or a metal silicide.When used to test for contaminants in a fabrication line, the previouslydescribed structure is created as follows. The insulating layer 12 isformed on the semiconductor substrate 10. This structure is then runthrough the fabrication, thereby exposing the insulating layer 12 to anycontaminants present. The conductive thin film 14 is deposited on theinsulating layer 12 and the conductive thin film 14 is annealed. Theannealing process may cause contaminants present in the insulating layer12 to migrate to the interface between the insulating layer 12 and thesemiconductor substrate 10.

A programmable beam laser, such as the Waferlase Wafer Marking Systemavailable from AB lasers (affiliated with Alltec GmbH and Baasel LaserTechnology GmbH) is used to pattern the conductive thin film 14. TheWaferlase Wafer Marking System uses a solid state diode pumpedyttrium-aluminum-garnet (YAG) laser with a beam size of 20 micrometers,a maximum power of 1.5 watts, a wavelength of 1064 nanometers and a linewidth of approximately 0.1 millimeters. Other lasers, such as an excimerlaser, may also be used.

The laser is programmed so that the beam will trace the desired shape ofthe patterned area on the surface of the conductive thin film. Thisshape may need to be programmed into the laser in consecutive segments.For example, in the absence of a program for patterning a circle,several programmable arcs may be used to create a circular pattern.

FIGS. 4 and 5 illustrate the lasing process. The programmed lasercreates a beam 20 which is focused on the conductive thin film 14. Thefocused laser beam 20 is cycled along outer boundary 22 to evaporate theconductive thin film 14 to create channel 24 and isolate patterned area23. By repeatedly cycling the laser along the outer boundary 22, thedeposition of conductive thin film 14 into channel 24 is substantiallyinhibited. After the channel 24 has been created, the laser is turnedoff. This may be done automatically by the patterning program after apredetermined number of cycles or manually by an operator who observesthe conductive thin film 14 to determine when an appropriate amount ofthe conductive thin film 14 has been removed. No particular atmosphereis required during the lasing process.

Ideally, only the conductive thin film 14 will be removed when creatinga C-V dot. While inadvertent removal of the insulating material alongthe bottom of channel 24 may occur, this will not prevent the capacitivestructure from functioning properly. If the thickness of insulatinglayer 12 is completely removed along the bottom of channel 24, the laserbeam may inadvertently remove a small portion of the underlying siliconin substrate 10. However, the amount of silicon removed will not besubstantial because silicon has a high transmittance which allows thelaser beam to pass through and because the laser beam loses focus whenit reaches the substrate

FIG. 6 shows a C-V plot for a capacitor created using the methodpreviously described. This capacitor comprises a silicon substrate, aninsulating layer of 1000 Å of silicon dioxide and a circular aluminumalloy (1% Si, 1/2% Cu) C-V dot with a diameter of 1260 microns. FIG. 7summarizes the measured parameters of this C-V dot.

An alternative embodiment of the present invention is shown in FIGS. 8through 10. In this embodiment, a focused laser beam 30 is applied to awafer consisting of a monocrystalline semiconductor substrate 31, aninsulating layer 32 and a conductive thin film 33. Again, thesemiconductor substrate 31 and insulating layer 32 are typically siliconand silicon dioxide, respectively. The conductive thin film 33 istypically metal, conductively doped polysilicon or a metal silicide. Thelaser beam 30 is programmed to trace a first boundary 34 and a secondboundary 35 on the conductive thin film 33. The laser beam 30 is appliedto first boundary 34 to remove the conductive thin film 33 existingalong this boundary and thereby form first channel 36. The laser beam 30is then applied to second boundary 35 to remove the conductive thin film33 along this boundary and thereby form second channel 37. The portionof conductive thin film 33 located within second boundary 35 can be usedas a C-V dot 38. This configuration provides for improved isolation andreduced leakage of the capacitive device. The second boundary 35 ispatterned last to avoid the possibility of conductive thin filmsplashing into second channel 37 during the patterning of first channel36. Conductive material in the second channel 37 is undesirable since itmay create a short circuit between the C-V dot 38 and the rest of theconductive thin film 33 or the semiconductor substrate 31.

While the invention has been described with reference to particularembodiments, this description is solely for the purpose of illustrationand is not to be construed as limiting the scope of the invention. Forexample, the C-V dot may be shaped as a circle, square, or any varietyof shapes by programming the path of the laser. Also, there may be morethan one additional outer boundary area used to provide improvedisolation the C-V dot. In addition, the same results may be obtained bymoving the substrate rather than the laser beam. Various modificationsand applications may be made by those skilled in the art withoutdeparting from the scope of the invention as defined in the followingclaims.

What is claimed is:
 1. A method of testing for contaminants in asemiconductor processing environment, said method comprising the stepsof:forming an insulating layer on a semiconductor substrate; forming alayer of conductive thin film on said insulating layer; defining aclosed outer boundary on a surface of said conductive thin film, saidclosed outer boundary defining an area of said conductive thin film tobe patterned; energetically removing said conductive thin film alongsaid closed outer boundary by applying a laser along said closed outerboundary, whereby said area of said conductive thin film is isolatedfrom the remainder of said conductive thin film; exposing the insulatinglayer and semiconductor substrate to the semiconductor processingenvironment; and applying a cyclic voltage to said area of saidconductive thin film.
 2. The method according to claim 1, wherein saidconductive thin film is a metal, polysilicon or a metal silicide.
 3. Themethod according to claim 1, wherein said conductive thin film consistsprimarily of aluminum.
 4. The method according to claim 1, wherein saidinsulating layer is silicon dioxide.
 5. The method according to claim 1,wherein said substrate is silicon.
 6. The method according to claim 1,wherein a computer program is used to control the application of saidlaser along said closed outer boundary.
 7. The method according to claim1, wherein said laser is a yttrium-aluminum-garnet laser.
 8. The methodaccording to claim 1, wherein said area of said conductive thin film isa circle.
 9. The method of claim 1, further comprising the stepsof:defining a closed insulating boundary on said surface of saidconductive thin film, wherein said closed insulating boundary laterallysurrounds said closed outer boundary; and energetically removing saidconductive thin film along said closed insulating boundary by applying alaser along said closed insulating boundary.
 10. A method of creating asemiconductor capacitor comprising the steps of:forming an insulatinglayer on a semiconductor substrate; forming a layer of conductive thinfilm on said insulating layer; defining a closed outer boundary on asurface of said conductive thin film, said closed outer boundarydefining an area of said conductive thin film to be patterned; defininga closed insulating boundary on said surface of said conductive thinfilm, wherein said closed insulating boundary laterally surrounds saidclosed outer boundary; energetically removing said conductive thin filmalong said closed insulating boundary by applying a laser along saidclosed insulating boundary; energetically removing said conductive thinfilm along said closed outer boundary by applying a laser along saidclosed outer boundary, whereby said area of said conductive thin film isisolated from the remainder of said conductive thin film.
 11. The methodaccording to claim 10, wherein said laser is applied along said closedinsulating boundary before said laser is applied along said closed outerboundary.
 12. The method according to claim 11, wherein a plurality ofsaid closed insulating boundaries are defined and said laser issequentially applied along each of said closed insulating boundaries toenergetically remove said conductive thin film along said closedinsulating boundaries.